A split four-stroke cycle internal combustion engine is described in, but not limited by, U.S. Pat. Nos. 6,543,225, 6,952,923 and 6,986,329. It includes at least one power piston and a corresponding expansion or power cylinder, and at least one compression piston and a corresponding compression cylinder. The power piston reciprocates through a power stroke and an exhaust stroke of a four-stroke cycle, while the compression piston reciprocates through an intake stroke and a compression stroke. A pressure chamber or crossover passage interconnects the compression and power cylinders, with one or more crossover inlet valves providing substantially one-way gas flow from the compression cylinder to the crossover passage, and one or more crossover outlet valves providing gas flow communication between the crossover passage and the power cylinder. In this patent application, crossover valves refer only to the crossover outlet, not inlet, valves. The engine further includes intake and exhaust valves on the compression and power cylinders, respectively. According to the referenced patents and other related developments, the split-cycle engine potentially offers many advantages in fuel efficiency, especially when integrated with an additional air or gas storage tank interconnected with the crossover passage, which makes it possible to operate the engine as an air hybrid engine. Relative to an electrical hybrid engine, an air hybrid engine can potentially offer as much, if not more, fuel economy benefits at much lower manufacturing and waste disposal costs.
To achieve the potential benefits, the air or air-fuel mixture in the crossover passage has to be maintained, for the entire four stroke cycle, at a predetermined firing condition pressure, e.g. approximately 18.6 bar (or 270 psi) per U.S. Pat. No. 6,543,225. The pressure may reach over 50 bar (735 psi) or higher, per U.S. Pat. No. 6,952,923, U.S. Pat. No. 6,986,329, a brochure entitled “Scuderi Air Hybrid Engine” distributed at SAE 2006 Congress by the Scuderi Group, LLC, and the May 2006 issue of European Automotive Design. Illustrated in graph 14 of FIG. 1 are, per US Patent Publication US2009/0038598-A 1, dynamic pressure profiles at the downstream end of the crossover passage and inside the power cylinder of a certain split-cycle engine. The ignition happens around 18 degrees ATDC (After Top Dead Center of the power cylinder). The crossover valve (XV) opens and closes at −5 degrees ATDC (or 5 degrees before TDC or BTDC) and 25 degrees ATDC, respectively, which presents a narrow opening window. Similarly tight timing is also presented in U.S. Pat. No. 6,952,923. Opening the crossover valve at or near TDC, when the power cylinder volume is at its minimum, helps reduce re-compression of the gas in the power cylinder and improves the efficiency. By opening several degrees before TDC, instead of exactly at or after TDC, it helps expand the valve opening window.
To seal against a persistently high pressure in the crossover passage, a practical crossover valve is most likely a poppet or disk valve with an outwardly (i.e. away from the power cylinder, instead of into it) opening motion as suggested in U.S. Pat. No. 4,170,970. Outward valve design is routinely implemented for applications with a high-pressure manifold, for example various compressor exhaust valves as illustrated in U.S. Pat. No. 4,253,805 and SAE Paper 2005-01-1884. In addition, outward opening design is desirable to deal with interference between an engine valve and the piston for any design with small combustion chamber as articulated in U.S. Pat. No. 6,952,923 (Column 14-Line 63 and Column 22-Line 33), especially when the compression ratio is greater than 80 to 1 as claimed by U.S. Pat. No. 6,952,923 (claim 3), which leaves practically no combustion chamber around TDC. Outward design is therefore further illustrated in figures in U.S. Pat. No. 4,170,970, No. 7,421,987 and No. 7,636,984 and in US Patent Applications 2008/0054205-A 1, 2009/0038598-A 1, 2009/0038599-A 1, 2009/0039300-A 1, 2009/0133648-A 1 and 2009/0044778-A 1.
When closed, the valve disk or head is pressured against the valve seat under the crossover passage pressure. To open the valve, an actuator has to provide a large opening force to overcome the pressure force on the valve head as well as the inertia. The opening pressure force is caused by the opening differential pressure dPo, which in FIG. 1 is about 35 bars. The differential pressure falls dramatically once the crossover valve opens because of a substantial pressure-equalization between the crossover passage and the power cylinder or a rapid rise in the power cylinder pressure. Similar trends in dynamic pressure profile and magnitude are found in U.S. Pat. No. 6,543,225. The pressure may reach over 50 bar (735 psi) or higher, per U.S. Pat. No. 6,952,923, U.S. Pat. No. 6,986,329, a brochure entitled “Scuderi Air Hybrid Engine” distributed at SAE 2006 Congress by the Scuderi Group, LLC, and the May 2006 issue of European Automotive Design.
For an engine valve, the flow area is approximately equal to the product of its perimeter and the valve lift. The opening force has to overcome, in addition to the spring preload if any, the pressure force that is equal to the differential pressure on the valve times the valve head area. The flow area and the opening pressure force are thus proportional to the diameter and the diameter to the second power, respectively. For higher power and better efficiency, it is a good practice to maximize the diameter or perimeter of intake valves, or crossover valves in split-cycle engines. This also entails two or more crossover valves to achieve reasonable total flow area while minimizing the pressure force. U.S. Pat. No. 6,952,923 discloses one design with four 13-mm crossover valves and another design with two 18.4-mm crossover valves, resulting in on each valve an opening force of 464 N and 931 N, respectively, under an opening differential pressure dPo of 35 bars. The opening differential pressure force in a conventional engine of the same volume displacement is typically 400 N for an exhaust valve and much lower for an intake valve. The design with four 13-mm crossover valves has a more tolerable opening force, but it adds too much structure complexity and cost penalty because of a large number of the valves involved. The design with two 18.4-mm crossover valves presents large opening force, challenging the corresponding valve actuator in areas of functional capability, durability, size, power consumption, etc. It is even a greater challenge if one desires lower flow resistance and thus larger valve diameter, considering that a conventional engine of the same volume displacement may have two 32-mm intake valves. A 32-mm crossover valve would have an opening force of 2815 N, challenging for any actuator, under a differential pressure of 35 bars.
Various efforts have been made to overcome the large opening force on a crossover valve. In U.S. Pat. No. 7,421,987 and US Patent Application 2008/0054205-A 1, one uses a combination of a spring bias force and a hydraulic force.
In U.S. Pat. No. 7,636,984 and US Patent Application 2009/0044778-A 1, one uses a pneumatic booster or pressure balance mechanism that entails at least one pneumatic chamber (in addition to or other than the crossover passage itself) or one pneumatic piston (in addition to or other than the crossover valve head itself) or both to counter the differential pressure.
In summary, a crossover valve actuator has to deal with large opening force while providing reasonable gas flow area.